1. Field of the Invention
The present invention relates to a supported catalyst useful in the catalytic purification of exhaust gases.
2. Description of the Prior Art
The catalysts which are used for the purification of exhaust gases discharged from an automible internal combustion engine and the like must be highly reactive over a wide range of temperatures because the catalysts are subjected to service conditions in which the chemical reaction-governing factors such as the volume and concentration of reactants or the working temperature cannot be maintained constant.
One general type of supported catalyst system which is useful for the purification of exhaust gases is composed of a catalyst carrier coated with an active substance and further coated with a catalytic material. In one embodiment of this type of catalyst a thin film of active alumina (principally .gamma.-alumina) with a large surface area has been employed as the active substance. However, .gamma.-alumina is susceptible to changes in crystal structure over long service lives at high temperatures. The changes which occur result in a decrease of specific surface area, substantial loss of catalytic activity with the passage of time and loss in durability. Moreover, .gamma.-alumina reacts with such catalyst poisons as compounds of lead, sulfur and phosphorous which are present in the exhaust gas. Thus .gamma.-alumina is not always a satisfactory catalyst component in terms of antitoxicity effects. Moreover, for a catalyst which is formed from an oxide of a metal such as nickel, iron or mixtures thereof supported as the catalytic substance on .gamma.-alumina, long service life of the catalyst at high temperatures causes the metal oxides to react with .gamma.-alumina, resulting in deterioration of catalyst activity. Therefore, a need continues to exist for a catalyst system of improved durability and service life for the purification of exhaust gases discharged from the internal combustion engine.